1. Field of the Invention
The present invention relates to a member for a semiconductor apparatus, and more particularly, it relates to a member for a semiconductor apparatus such as a circuit substrate, which must be of high thermal conductivity to be mounted with a semiconductor device of high calorific power such as a high-power transistor or a laser diode.
2. Description of the Prior Art
A member for a semiconductor apparatus to be mounted with a semiconductor device is generally formed by an insulating member and a radiating member joined to the insulating member. For example, such a member for a semiconductor apparatus is formed by an insulating substrate to be provided thereon with a semiconductor device and a radiating substrate joined to the back surface of the insulating substrate by soldering through silver solder or the like. In this case, generally required for the insulating substrate are high electric insulability for insulation from the semiconductor device, high mechanical strength and high thermal conductivity for dissipating heat generated from the semiconductor device. The radiating substrate must have high thermal conductivity similarly to the insulating substrate, while its thermal expansion coefficient must be approximate to those of materials forming a semiconductor substrate, the insulating substrate and the like.
In general, alumina (Al.sub.2 O.sub.3) is selected as a material satisfying the aforementioned properties for forming the insulating substrate employed in such a member for a semiconductor apparatus. However, although alumina is excellent in electric insulability and mechanical strength, its heat dissipation property is inferior due to small thermal conductivity of 17 Wm.sup.-1 K.sup.-1. Thus, it is improper to carry a field-effect transistor (FET) of high calorific power, for example, on an alumina substrate. In order to carry a semiconductor device of high calorific power, another type of insulating substrate is prepared by beryllia (BeO) having high thermal conductivity of 260 Wm.sup.-1 K.sup.-1, whereas beryllia is toxic and hence it is troublesome to take safety measures in employment of such an insulating substrate.
The radiating substrate is generally prepared by a material satisfying the aforementioned properties, which material is selected from metal materials such as various types of copper alloys, copper-tungsten alloys and copper-molybdenum alloys. For example, Japanese Patent Laying-Open Gazette No. 21032/1984 discloses a substrate of high thermal conductivity for carrying a semiconductor device, the material of which is prepared by mixing 2 to 30 percent by weight of copper into tungsten or molybdenum. This substrate is employed as a radiating substrate which is suitably joined to an alumina substrate having inferior heat dissipation property, and difference in thermal expansion coefficient between the same and alumina is relatively small. Thus, this prior art example is insufficient in heat dissipation property, which is required entirely over a substrate for carrying a semiconductor device.
In recent years, nontoxic aluminum nitride (AlN) has generated great interest as a material for such an insulating substrate for carrying a semiconductor device of high calorific power because of its high thermal conductivity of about 200 Wm.sup.-1 K.sup.-1, which value is substantially equal to that of beryllia, as well as its electric insulability and mechanical strength which are equivalent to those of alumina.
However, when an aluminum nitride substrate provided with a metallized layer is soldered by a soldering metal such as gold solder or silver solder, for example, to a generally employed radiating substrate of a copper-tungsten alloy or copper-molybdenum alloy containing 10 to 25 percent by weight of copper, the aluminum nitride substrate may be cracked or the radiating substrate of the copper-tungsten alloy or the copper-molybdenum alloy may be warped.
Such a phenomenon results from thermal stress caused of difference in thermal expansion coefficient between the copper-tungsten alloy or the copper-molybdenum alloy and the aluminum nitride during a cooling step after soldering, which is performed at a temperature of 500.degree. to 950.degree. C. This thermal stress may conceivably be left in the aluminum nitride substrate as tensile residual stress, to crack the aluminum nitride substrate and/or warp the radiating substrate of the copper-tungsten alloy or the copper-molybdenum alloy.
When an aluminum nitride substrate is joined to a radiating substrate of a copper-tungsten alloy or a copper-molybdenum alloy by cold soldering or soldering, the aluminum nitride substrate or an interface between the same and a metallized layer is cracked by a thermo-cycle test (-55.degree. C. to +150.degree. C., 1000 cycles) or a thermal shock test. The result indicates a significant problem in practice, even if neither a warp nor a crack is recognized upon joining.
In a sample of an aluminum nitride substrate joined to a radiating substrate of a copper-tungsten alloy or a copper-molybdenum alloy by silver soldering, thermal fatigue or thermal stress was caused in a thermo-cycle test or a thermal shock test due to difference in thermal expansion coefficient between the radiating substrate of the copper-tungsten alloy or the copper-molybdenum alloy and the aluminum nitride substrate, similarly to the above. Such a problem of thermal stress or thermal fatigue is aggravated with increase in junction area.
Thermal expansion coefficients of the copper-tungsten alloy or the copper-molybdenum alloy having the aforementioned composition and aluminum nitride are 6.5 to 10.times.10.sup.-6 /K and 4 to 5.times.10.sup.-6 /K respectively, within a range of the room temperature to about 950.degree. C. Further, these materials, having high Young's modulus of 27,000 to 35,000 Kg/mm.sup.2 and 35,000 to 37,000 Kg/mm.sup.2 respectively, are hardly plastically deformed. Thus, when the copper-tungsten alloy or the copper-molybdenum alloy of the aforementioned composition and aluminum nitride are joined with each other by soldering, large thermal stress is conceivably caused in a cooling step.